Automatic parking brakes (APB) may include an operating element, such as a button, using which the parking brake may be locked or disengaged. Upon operation of the operating element, a control unit connected thereto recognizes the parking brake command and accordingly activates a final control element, such as a hydraulic pump or an electric motor, to build up brake force on the wheels of the vehicle or to disengage the brake.
It is understood that there are automatic parking brakes (APB) in which electric motors are located directly on the brake caliper of the wheel brakes (so-called “motor-on-caliper”). The electric motors act via a gearing, such as a spindle drive, directly on the brake piston of the hydraulic brake system and therefore can apply or disengage the service brake. The electric motors are typically dimensioned in such a way that their clamping force is sufficient to hold a vehicle on slopes of at least 20% without energy, as required by law.
In addition, it is understood that there are parking brake systems in which the electric motors are designed more weakly. In order to nonetheless be able to achieve the required clamping force, the electric motors are supported by the hydraulic pump of the hydraulic service brake system. During an application procedure of the parking brake, the electric motors are first operated and the hydraulic pump is switched on after a specific amount of time. The clamping force may thus be amplified up to the required value.
In the last-mentioned APB systems having hydraulic support, however, it is problematic that the total clamping force can only be determined relatively imprecisely or only with high sensory outlay.